Watch Salman Khan explain the next big challenge in online education.

In a TED talk, Salman Khan of Khan Academy explains how his homespun educational videos can be part of a broader move to flip the traditional classroom experience: Kids watch lectures online at home, then do their "homework" in the classroom.

Want to learn calculus? In this video, the first in Khan Academy's series of more than 100 on the subject, Khan introduces the idea of limits.

It's OK if math isn't your thing—there are Khan Academy videos on every imaginable subject. Here, Khan embarks on a concise overview of "everything that's happened to Earth since it came into existence."